Figure 1.

Possible Met and Trp oxidation products occurring in proteins. Chiral centers are indicated by an asterisk (a red asterisk represents the chiral centers generated upon the transformation of the natural residues). In the case of HPI, two stereoisomers are expected, which correspond to the cis or trans configurations of the OH group with respect to the α-carbonyl within the pyrrole ring, as previously defined by Ronsein et al.²⁹ In the case of N-formylkynurenine (NFK), two species can form, due to the cis and trans isomerization of the N-aryl-formamide moiety (indicated by ‡).